Quickest way to create a color gradient texture
You can find under the Add >> Texture >> Gradient Texture if you are using the Node Editor:



Or in the Material panel as shown below



Using the node editor is probably easiere because it helps to immediately figure out which node is doing what, the relations between the various components of the shader and quickly navigate the nodetree, while with the menus would be more tricky.

Down below you can see an example of usage of the gradient texture to mix two colors. On the top the node editor and on ther righit side the corrispective menu's view of the same material.